1. Field of the Invention
The present invention is directed generally to micro-electromechanical systems (MEMS) accelerometers.
2. Background Art
An accelerometer is a device that measures an acceleration. Using MEMS technology, an accelerometer can be fabricated on a (silicon) substrate. Accelerometer elements constructed using MEMS include structures similar to a standard accelerometer: a proof-mass, restoring springs, a displacement transducer, some form of damping, and a case to which everything is attached.
For example, FIG. 1A illustrates an example accelerometer with a few features specific to a subset of MEMS accelerometers. Shown is a proof mass 1, pair of restoring springs 2, a case 3, displacement transducers 4 and 5, and a damper 6. Case 3, although drawn as two separate pieces and shown in cross-section, is assumed to be constructed as one effectively rigid body. Displacement transducers 4 and 5 are shown as differential capacitance transducers, but could be piezoelectric transducers or some other form of transducer, as would be apparent to a person skilled in the relevant art(s).
In response to a horizontal acceleration to the left, proof mass 1 will move to the right. As a result of this motion, the capacitance of displacement transducer 5 increases while the capacitance of displacement transducer 4 decreases. The difference in capacitance between displacement transducers 4 and 5 provides a measure of the relative motion of proof mass 1 with respect to case 3, and hence a measure of the acceleration to which proof mass 1 is being subjected. Any ringing of the accelerometer due to sudden acceleration changes is damped by damper 6.
To provide the necessary electrical circuitry, such a MEMS accelerometer can be wire bonded to an Application Specific Integrated Circuit (ASIC). An electrical model for the accelerometer of FIG. 1A is shown in FIG. 1B. The differential capacitance between capacitors 4 and 5 can be measured in many ways. Typically square wave carrier signals that are 180 degrees out of phase are sent into terminals 7 and 8. These carrier signals are referred to simply as carrier 1 and carrier 2, respectively, in this discussion.
The magnitude of the square waves depends on the ASIC technology used; however, voltages in the 1.8 to 5V range are typical. As the square wave voltages transition from high to low or low to high, a charge must flow through terminal 9. If the two sides are balanced, no net charge flows. By measuring the amount of charge that flows through terminal 9, one has a measure of the capacitance difference and hence the acceleration to which the device is being subjected. Terminal 9, the terminal on the ASIC that integrates the charge, is referred to as the charge-in pad. Multiple sensors on the same MEMS die can share the carrier signals. For example an X sensor and a Y sensor can both use carriers 1 and 2 in the capacitance measurements; however, a separate charge-in connection is necessary for each sensor direction.
The MEMS accelerometer and ASIC are packaged in a packaging unit. Consequently, a full accelerometer based on MEMS is typically constructed of three components: (1) a MEMS element that senses acceleration, (2) electronics included in an ASIC that transduces the MEMS element's response to acceleration into an electronic signal, and (3) a package that houses the first and second components. A problem with current MEMS accelerometers is that they are temperature and package sensitive. That is, the detection of an acceleration by a MEMS accelerometer may be affected by changes in temperature and/or by stresses imposed on the packaging unit.
Therefore, what is needed is an improved MEMS accelerometer that is less temperature and package sensitive. In addition, the improved MEMS accelerometer should be configured to occupy as little of an area of the substrate as possible to thereby minimize the overall size of the accelerometer.